Re: And the Largest Theropod Is....

["Mickey Mortimer" <Mickey_Mortimer111@msn.com>]

Tim Williams wrote-

> Did Stromer provide linear measurements for *all* the elements, including
> the cranial material?   This could be important, because (as I'm sure you
> know) the association of all this material into a single species has been
> questioned.  Oliver Rauhut, for one, has suggested that the holotype for
_S.
> aegyptiacus_ is a composite.  But, if the dentary and vertebrae both come
> from a stupendously-sized theropod, it would bolster the case that the
> holotype probably represents one individual, and therefore a single
species.
>   While it is possible that two 17m-long theropods might turn up in one
> site, it must be rated as highly unlikely.

Indeed he did, the preserved dentary is 750 mm long, and we both estimate
the mandibular length at 1.2 meters (Stromer did pretty good for not having
any other spinosaur material).  And assuming Sereno et al.'s skeletal
reconstruction of Suchomimus is correct, this would correspond to a 9.8
meter long spinosaurid.  However, there are a few things to consider before
we start thinking the cranial and vertebral material don't belong together.
First, none of the cranial material of Suchomimus was found with postcrania,
so exactly how large its head was compared to its body is unknown.  Sereno
presumedly scaled approximately based on Baryonyx's holotype.  Another
possibility is that Spinosaurus' head was comparatively shorter than
Suchomimus'.  This is probable due to the greatly elongated snout in the
latter.  In quantified form, Suchomimus' mandibular depth is 19% of its
dentary length, while Spinosaurus' is 32%.  So assuming mandibular depth
(not length) is what scales with body size, we would expect Suchomimus to
have a head 1.7 times the length of a similarly sized Spinosaurus.  And
scaling the Spinosaurus holotype mandible up by this amount results in an
estimated length of 16.5 meters.  Pretty close to the 17.4 meters I
estimated from the vertebrae.
And if that's true, MNHN SAM 124 would come from an even larger individual.
Its skull is estimated at 2 meters long after all.  Scaling to the holotype
would result in a total length of ~28 meters.  Taquet and Russell say this
specimen is an adult based on the closed premaxillary sutures.  Was this the
size of adult Spinosaurus? An enormous 28 meter animal weighing at least 50
tons, perhaps up to 80.  Intriguing, no?

> P.S. Nice work, Mickey.

Thanks!

> Would _Richardoestesia_ be bigger than a 300-lb ostrich-sized
> _Gargantuavis_?

What a difficult question.  Gargantuavis seemingly weighed 140 kg, but
estimating Richardoestesia's mass is tricky.  The dentary really is quite
elongate, perhaps suggesting a solution akin to Suchomimus vs. Spinosaurus
above.  So, comparing the dentary depth to Sinornithosaurus (which
richardoestesiids seem to resemble the most), one sees it was about 1.9
times larger.  Of course, depth varies slightly along the dentary, so this
estimate is only tentative.  This would result in a length of about two
meters for Richardoestesia's holotype.  Using Deinonychus' femoral length to
scale the mass estimate (due to probable differences in tail proportions)
results in a mass of about 45 kg.  Gargantuavis would keep its title as
largest Mesozoic bird, assuming the many generalities I made for this
estimate are correct, of course. :-)

James Aronis wrote-

> I remember reading that the claw of _Megaraptor namunhuaiquii_ was
> considerably larger than the same claw belonging to _Utahraptor_. Given
> that nowhere near a complete skeleton of the latter has been discovered,
> could we extrapolate which was the larger animal?

The known pedal ungual II of Megaraptor is indeed much larger (~1.6 times)
than the described pedal unguals of Utahraptor.  So the holotype of
Megaraptor was probably about that much larger than the holotype of
Utahraptor (known from a pedal ungual), and those other described
individuals known from pedal unguals (which are all smaller than the
holotype).  I estimated the "~1.6 times" figure from their proximal
articular surfaces, so variation in ungual size (as seen between Adasaurus
and Deinonychus for instance) should not be a factor.  Of the new material,
a reported 565 mm long femur probably came from an animal about 6.1 meters
long, so still smaller than Megaraptor's holotype.  Other described material
(premaxilla, three caudal vertebrae, tibia) seems to be about this size or
smaller as well.  So only the undescribed caudals support Utahraptor
individuals larger than Megaraptor's holotype for the time being.  We'll
just have to wait for publication to see if any of the other 190 undescribed
elements are from such large individuals.  Another possibility is that Britt
et al. erred in their statement regarding the caudals.  An SVP presentation
is hardly a peer-reviewed paper after all, many things can change before
publication.

Mickey Mortimer